Take some of your hay in and have it analyzed by your vet to see where it is at.
And you might look into a feed with less carbs/molasses/corn, if she is getting that. And just because you haven't changed her feed, doesn't mean it is reacting same way now as it did years ago, OR that company didn't tinker with the formula and it is very different.
But feed/hay will cause some major changes in horses, and is one of the easiest things to fix too.
I've known TWH/Racking Horses that got like this on crimped oats, and a QH that would be hanging from stall rafters after one flake of peanut hay. Just climbing out of his skin.
Worth a try. Keep a notebook of what you are trying and what you are seeing with her, so you can have a record of changes.